a) Field of the Invention
The invention is directed to an arrangement for generating a pulsed laser beam of high average output, particularly for generating a hot plasma which emits extreme ultraviolet (EUV) radiation.
b) Description of the Related Art
Pulsed lasers with a high average output and good beam quality are needed for certain applications such as a pumped light source of a laser-induced plasma for the generation of EUV radiation. The high average output is achieved with short pulse lengths by high repetition rates in the kilohertz range.
However, some laser types such as TEA-CO2 lasers, excimer lasers and also some Nd-YAG lasers with elements for phase conjugation which deliver pulsed radiation have a limited repetition frequency. For example, the maximum repetition frequency of a TEA-CO2 laser is typically 250 Hz with a pulse length of only 100 ns and a pulse energy between 1 J and 20 J per individual laser.
In the prior art, DE 199 35 404 A1 discloses an illumination system for wavelengths of less than 193 nm, particularly for EUV lithography, which has a plurality of light sources and a mirror arrangement for generating secondary light sources with a plurality of mirrors which are divided into raster elements. The aim here is to illuminate the exit pupil of the illumination system by a plurality of light sources as homogeneously as possible up to a determined filling degree. For this purpose, a honeycomb plate is provided with a plurality of pyramids for joining the partial light source pupils together in an additive (non-overlapping) manner or for mixing the light of the partial light sources illuminating the exit pupil. In this way, it is only possible to achieve a local increase in the geometric flux (etendue) in the exit pupil of the total system with a considerable expenditure on optical alignment. Pulse excitation with increased frequency is not described.
Further, U.S. Pat. No. 5,818,546 is cited as non-generic prior art, wherein a device is disclosed for generation of an image in an image surface. Using projection optics, a radiation source generates an image in that at least one image series is formed by illuminating adjacent image spots during an image forming cycle and the image forming cycle is constantly repeated. Image generation is carried out by a succession of semiconductor emitters that are imaged on metrically organized image spots of the image surface by a polygonal mirror system and/or tilting mirror system. The intensity of the individual semiconductor emitters is controllable in order to display different contents for image generation in the human eye. An exact synchronization with the position of the mirror surfaces is provided by the pulsed control of the semiconductor emitters. Blurring effects of the image points caused by a slight movement of the mirror surfaces during the controlling of the light emitters are wanted and, therefore, no measures are undertaken to suppress them.